Method for updating route in network, network device, and system

ABSTRACT

This application provides a method for updating a route in a network. The first network device sends a first LSA packet to a third network device, so that the third network device generates a first route whose destination address is a second IP address, where a next-hop IP address of the first route is the IP address of the first network device, and the second IP address belongs to the IP network segment corresponding to the first IP address which is an IP address of the first network device. The first network device sends a second LSA packet to the third network device when determining that switching needs to be performed on a next hop of a route in the third network device, whose destination address belongs to the IP network segment, and whose next-hop IP address is the first IP address.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/530,203, filed on Aug. 2, 2019, which is a continuation ofInternational Application No. PCT/CN2018/073643, filed on Jan. 22, 2018,which claims priority to Chinese Patent Application No. 201710064404.0,filed on Feb. 4, 2017. All of the aforementioned applications are herebyincorporated by reference in their entireties.

TECHNICAL FIELD

This application relates to the field of communications technologies,and in particular, to a method for updating a route in a network, anetwork device, and a system.

BACKGROUND

An Open Shortest Path First (English: Open Shortest Path First, OSPF)protocol is used in a single autonomous system (English: AutonomousSystem, AS). In an entire network, each router sets up an OSPF neighborrelationship and sends, to all neighbors, a link state advertisement(English: Link State Advertisement, LSA) packet generated by the router.The OSPF advertises a network interface state between the routers andsets up a link-state database. Each router calculates a shortest-pathtree based on the link-state database, to generate a routing table.

In an OSPF network area, an autonomous system boundary router (English:Autonomous System Boundary Router, ASBR) leads a route of an externalnetwork to the OSPF, generates an AS external LSA (English:AS-external-LSA) packet, and sends the AS external LSA packet to arouter in the OSPF network area. Generally, the router in the OSPFnetwork area communicates with the external network by using the ASBR.In the related art, when the router in the OSPF network area istriggered to update a route, the ASBR needs to age, one by one,AS-external-LSA packets advertised by the ASBR, and advertises aginginformation to its connected router in the OSPF network area, to updatethe route.

However, in the prior art, if the ASBR is connected to routers in aplurality of network segments, the ASBR cannot flexibly enable itsconnected router in the OSPF network area to update a route withoutchanging a link from the ASBR to the external network. If there are arelatively large quantity of routers in the network area, a maximumtransmission unit (English: Maximum Transmission Unit, MTU) in aninterface connecting the ASBR and the routers in the OSPF network areais 1500 bytes, and a bandwidth of 0.373 bps needs to be occupied forsending one AS-external-LSA packet, the ASBR needs to send at least 2700packets to notify its connected routers in the OSPF network area of allaging information when 100,000 routes need to be updated on the routersin the OSPF network area. In this process, a large quantity of networkbandwidths need to be occupied, and a large amount of time needs to beconsumed. Consequently, it takes an excessively long time to update theroutes, and packet loss occurs.

SUMMARY

Embodiments of this application provide a method for updating a route ina network, a network device, and a system, to resolve prior-arttechnical problems that a route update occupies a large quantity ofbandwidths and consumes a large amount of time.

According to a first aspect, an embodiment of this application providesa method for updating a route in a network, where the network includes afirst network device, a second network device, and a third networkdevice, the third network device is connected to the first networkdevice and the second network device, and the method includes:

setting up, by the first network device, a correspondence between an IP(English: Internet Protocol, Internet Protocol) network segment and afirst IP address, where the first IP address is an IP address of thefirst network device; sending, by the first network device, a first LSApacket to the third network device, where the first LSA packet includesthe second IP address, the first LSA packet is used to trigger the thirdnetwork device to generate a first route whose destination address isthe second IP address, the second IP address belongs to the IP networksegment, and a next-hop IP address of the first route is the first IPaddress; and sending, by the first network device, a second LSA packetto the third network device when the first network device determinesthat switching needs to be performed on a next hop of a route that isstored in the third network device, whose destination IP address belongsto the IP network segment, and whose next-hop IP address is the first IPaddress, where the second LSA packet includes the first IP address, thesecond LSA packet is used to trigger the third network device to update,from the first IP address to a third IP address, the next-hop IP addressof the first route, and the third IP address is an IP address of thesecond network device.

In the foregoing solution, the first network device sends the first LSApacket to the third network device, so that the third network devicegenerates the first route whose destination address is the second IPaddress, where the next-hop IP address of the first route is the IPaddress of the first network device, and the second IP address belongsto the IP network segment. The first network device sends the second LSApacket to the third network device when determining that switching needsto be performed on the next hop of the route that is in the thirdnetwork device, whose destination address belongs to the IP networksegment, and whose next-hop IP address is the first IP address, so thatthe third network device switches, to the IP address of the secondnetwork device, the next-hop IP address of the route whose next-hop IPaddress is the first IP address. In this way, one LSA packet can be sentto enable the third network device to switch, from the first networkdevice to the second network device, the next hop of the route belongingto the IP network segment, and there is no need to send an LSA packetfor each to-be-updated route. This avoids occupation of excessivebandwidths during a route update and improves a speed of the routeupdate.

In a possible design, the method further includes:

sending, by the first network device, a third LSA packet to the thirdnetwork device, where the third LSA packet includes the first IPaddress, and the third LSA packet is used to trigger the third networkdevice to generate a route whose destination address is the first IPaddress; and that the second LSA packet is used to trigger the thirdnetwork device to update, from the first IP address to a third IPaddress, the next-hop IP address of the first route includes:

the second LSA packet is used to instruct the third network device todelete the route whose destination address is the first IP address, totrigger the third network device to update the first route whosenext-hop IP address is the first IP address, so that the next-hop IPaddress of the first route is updated from the first IP address to thethird IP address, where the third network device stores a route whosedestination address is the third IP address.

In the foregoing solution, the first network device sends the second LSApacket to the third network device when determining that switching needsto be performed on the next hop of the route that is in the thirdnetwork device, whose destination address belongs to the IP networksegment, and whose next-hop IP address is the first IP address, so thatthe third network device switches, to the IP address of the secondnetwork device, the next-hop IP address of the route whose next-hop IPaddress is the first IP address. In this way, one LSA packet is sent toenable the next hop of the route belonging to the IP network segment toswitch from the first network device to the second network device, andthere is no need to send an LSA packet for each to-be-updated route.This avoids occupation of excessive bandwidths during a route update andimproves a speed of the route update.

In a possible design, the first IP address is an IP address of a logicalinterface of the first network device, and a forwarding address in thefirst LSA packet is the first IP address.

In the foregoing solution, the first IP address may be the IP address ofthe logical interface of the first network device.

In a possible design, after the sending, by the first network device,the second LSA packet to the third network device, the method furtherincludes:

sending, by the first network device to the third network device whenthe first LSA packet is stored in a link-state database for a timelength reaching a maximum age (English: MaxAge), a packet for deletingthe first LSA packet, so that the third network device deletes thestored first LSA packet.

In the foregoing solution, after sending the second LSA packet, thefirst network device may continue to perform an aging operation on thefirst LSA packet, to delete the first LSA packet, thereby saving storagespace.

According to a second aspect, an embodiment of this application providesa route switching method in a network, where the network includes afirst network device, a second network device, and a third networkdevice, the third network device is connected to the first networkdevice and the second network device, a correspondence between an IPnetwork segment and a first IP address is set up by the first networkdevice, and the first IP address is an IP address of the first networkdevice, and the method includes:

receiving, by the third network device, a first LSA packet sent by thefirst network device, where the first LSA packet includes a second IPaddress, and the second IP address belongs to the IP network segment;and generating, by the third network device based on the first LSApacket, a first route whose destination address is the second IPaddress, and a next-hop IP address of the first route is the first IPaddress; receiving, by the third network device, a second LSA packetsent by the first network device, where the second LSA packet is sent tothe third network device when the first network device determines thatswitching needs to be performed on a next hop of a route that is storedin the third network device, whose destination IP address belongs to theIP network segment, and whose next-hop IP address is the first IPaddress, and the second LSA packet includes the first IP address; andupdating, by the third network device from the first IP address to athird IP address, the next-hop IP address of the first route based onthe second LSA packet, where the third IP address is an IP address ofthe second network device.

In the foregoing solution, the third network device rapidly switches, tothe IP address of the second network device based on the second LSApacket, the next hop of the route whose destination IP address belongsto the IP network segment and whose next-hop IP address is the first IPaddress. This avoids occupation of excessive bandwidths during a routeupdate and improves a speed of the route update.

In a possible design, the method further includes:

receiving, by the third network device, a third LSA packet sent by thefirst network device, where the third LSA packet includes the first IPaddress; and generating, by the third network device based on the thirdLSA packet, a route whose destination address is the first IP address;and the updating, by the third network device from the first IP addressto a third IP address, the next-hop IP address of the first route basedon the second LSA packet includes:

receiving, by the third network device, a fourth LSA packet sent by thesecond network device, where the fourth LSA packet includes the third IPaddress; and generating, by the third network device based on the fourthLSA packet, a route whose destination address is the third IP address;and

updating, by the third network device based on the second LSA packet,the first route whose next-hop IP address is the first IP address, sothat the next-hop IP address of the first route is updated from thefirst IP address to the third IP address.

In a possible design, the first IP address is an IP address of a logicalinterface of the first network device, and a forwarding address in thefirst LSA packet is the first IP address.

According to a third aspect, an embodiment of this application providesa first network device, where the first network device, a second networkdevice, and a third network device are in a same network, the thirdnetwork device is connected to the first network device and the secondnetwork device, and the first network device includes:

a processing unit, configured to set up a correspondence between an IPnetwork segment and a first IP address, where the first IP address is anIP address of the first network device; and

a sending unit, configured to send a first LSA packet to the thirdnetwork device, where the first LSA packet includes the second IPaddress, the first LSA packet is used to trigger the third networkdevice to generate a first route whose destination address is the secondIP address, the second IP address belongs to the IP network segment, anda next-hop IP address of the first route is the first IP address.

The processing unit is configured to determine whether switching needsto be performed on a next hop of a route that is stored in the thirdnetwork device, whose destination IP address belongs to the IP networksegment, and whose next-hop IP address is the first IP address.

The sending unit is configured to send a second LSA packet to the thirdnetwork device when the processing unit determines that switching needsto be performed on the next hop of the route that is stored in the thirdnetwork device, whose destination IP address belongs to the IP networksegment, and whose next-hop IP address is the first IP address, wherethe second LSA packet includes the first IP address, the second LSApacket is used to trigger the third network device to update, from thefirst IP address to a third IP address, the next-hop IP address of thefirst route, and the third IP address is an IP address of the secondnetwork device.

In a possible design, the following is further included:

the sending unit is further configured to send a third LSA packet to thethird network device, where the third LSA packet includes the first IPaddress, and the third LSA packet is used to trigger the third networkdevice to generate a route whose destination address is the first IPaddress; and correspondingly,

the second LSA packet is used to instruct the third network device todelete the route whose destination address is the first IP address, totrigger the third network device to update the first route whosenext-hop IP address is the first IP address, so that the next-hop IPaddress of the first route is updated from the first IP address to thethird IP address, where the third network device stores a route whosedestination address is the third IP address.

According to a fourth aspect, an embodiment of this application providesa first network device. The first network device includes a processor, anetwork interface, and a memory. The memory may be configured to storeprogram code and data of the network device. The processor is configuredto invoke a program instruction from the memory to perform the method inthe designs in the foregoing aspects. For a specific execution step,refer to the foregoing aspects. Details are not described herein again.

According to a fifth aspect, an embodiment of this application providesa third network device, where the third network device, a first networkdevice, and a second network device are in a same network, the thirdnetwork device is connected to the first network device and the secondnetwork device, a correspondence between an Internet Protocol IP networksegment and a first IP address is set up by the first network device,the first IP address is an IP address of the first network device, andthe third network device includes:

a receiving unit, configured to: receive a first LSA packet sent by thefirst network device, where the first LSA packet includes a second IPaddress, the second IP address belongs to the Internet Protocol IPnetwork segment; and receive a second LSA packet sent by the firstnetwork device, where the second LSA packet is sent to the third networkdevice when the first network device determines that switching needs tobe performed on a next hop of a route that is stored in the thirdnetwork device, whose destination IP address belongs to the IP networksegment, and whose next-hop IP address is the first IP address, and thesecond LSA packet includes the first IP address; and

a processing unit, configured to generate, based on the first LSApacket, a first route whose destination address is the second IPaddress, and a next-hop IP address of the first route is the first IPaddress, and update, from the first IP address to a third IP address,the next-hop IP address of the first route based on the second LSApacket, where the third IP address is an IP address of the secondnetwork device.

In a possible design, the following is further included:

the receiving unit is further configured to receive a third LSA packetsent by the first network device, where the third LSA packet includesthe first IP address; and

the processing unit is further configured to generate, based on thethird LSA packet, a route whose destination address is the first IPaddress;

the receiving unit is further configured to receive a fourth LSA packetsent by the second network device, where the fourth LSA packet includesthe third IP address; and

the processing unit is further configured to generate, based on thefourth LSA packet, a route whose destination address is the third IPaddress.

According to a sixth aspect, an embodiment of this application providesa third network device. The third network device includes a processor, anetwork interface, and a memory. The memory may be configured to storeprogram code and data of the network device. The processor is configuredto invoke a program instruction from the memory to perform the method inthe designs in the foregoing aspects. For a specific execution step,refer to the foregoing aspects. Details are not described herein again.

According to a seventh aspect, an embodiment of this applicationdiscloses a network system, including: the first network devicedisclosed in the third aspect or the fourth aspect, a second networkdevice, and the third network device disclosed in the fifth aspect orthe sixth aspect.

The third network device is connected to the first network device andthe second network device.

The first network device is configured to: set up a correspondencebetween an IP network segment and a first IP address, where the first IPaddress is an IP address of the first network device; send a first LSApacket to the third network device, where the first LSA packet includesthe second IP address, the first LSA packet is used to trigger the thirdnetwork device to generate a first route whose destination address isthe second IP address, the second IP address belongs to the IP networksegment, and a next-hop IP address of the first route is the first IPaddress; and send a second LSA packet to the third network device whendetermining that switching needs to be performed on a next hop of aroute that is stored in the third network device, whose destination IPaddress belongs to the IP network segment, and whose next-hop IP addressis the first IP address, where the second LSA packet includes the firstIP address, the second LSA packet is used to trigger the third networkdevice to update, from the first IP address to a third IP address, thenext-hop IP address of the first route, and the third IP address is anIP address of the second network device.

The third network device is configured to: receive the first LSA packetsent by the first network device; generate, based on the first LSApacket, the first route whose destination address is the second IPaddress, where the next-hop IP address of the first route is the firstIP address; receive the second LSA packet sent by the first networkdevice; and update, from the first IP address to the third IP address,the next-hop IP address of the first route based on the second LSApacket, where the third IP address is the IP address of the secondnetwork device.

The second network device is configured to forward a packet sent by thethird network device.

According to an eighth aspect, an embodiment of this applicationprovides a non-volatile computer readable storage medium configured tostore a computer program, where the computer program includes aninstruction used to perform the method in the first aspect, the secondaspect, any possible design in the first aspect, or any possible designin the second aspect.

BRIEF DESCRIPTION OF DRAWINGS

To describe the technical solutions in the embodiments of thisapplication more clearly, the following briefly describes theaccompanying drawings required for describing the embodiments or theprior art. Apparently, the accompanying drawings in the followingdescription merely show some embodiments of this application, andpersons of ordinary skill in the art may derive other drawings from theprovided accompanying drawings without creative efforts.

FIG. 1 is a schematic structural diagram of a network system accordingto an embodiment of this application;

FIG. 2 is a schematic diagram of a format of a Router-LSA packetaccording to an embodiment of this application;

FIG. 3 is a schematic diagram of a format of an AS-external-LSA packetaccording to an embodiment of this application;

FIG. 4 is a schematic flowchart of a method for updating a route in anetwork according to an embodiment of this application;

FIG. 5 is a schematic structural diagram of a first network deviceaccording to an embodiment of this application;

FIG. 6 is a schematic structural diagram of a third network deviceaccording to an embodiment of this application;

FIG. 7 is a schematic structural diagram of another first network deviceaccording to an embodiment of this application;

FIG. 8 is a schematic structural diagram of another third network deviceaccording to an embodiment of this application; and

FIG. 9 is a schematic structural diagram of a network system accordingto an embodiment of this application.

DESCRIPTION OF EMBODIMENTS

The following describes the embodiments of the present invention withreference to accompanying drawings.

In the embodiments, claims, and accompanying drawings of thisapplication, the terms “first”, “second”, “third”, “fourth”, and thelike are intended to distinguish between different objects, but not toindicate a particular order. In addition, the terms “include” and “have”are not exclusive. For example, the process, method, system, product, ordevice that includes a series of steps or units is not limited to thelisted steps or units, but may further include a step or a unit that isnot listed.

The network device mentioned in the embodiments of this application maybe a router or may be a network device with a same or similar function,such as a switch.

The embodiments of this application are described by using an OSPFprotocol as an example. The OSPF protocol may also be referred to as alink state routing protocol. A network device configured with the OSPFprotocol may obtain all information of other network devices to create anetwork topology structure, and select, in the topology structure, anoptimal path that reaches each destination network. In an entire OSPFnetwork, each router sends, to all neighbors by setting up a neighborrelationship, an LSA packet generated by the router. The OSPF advertisesa network interface state between the routers and sets up a link-statedatabase. In the OSPF protocol, a topology structure in an OSPF networkarea may be described by using the link-state database, and networkdevices in a same OSPF network area have a same link-state database.Each router in the OSPF network calculates a shortest-path tree based onthe link-state database, to generate a routing table.

In the OSPF network area, a router in the area communicates with anexternal network by using a router that acts as an ASBR. A route updateis required when a network topology structure of the router in the areachanges. The route update indicates updating content of the routingtable after the network topology structure changes. A route convergenceis caused by the change of the network topology and is a behavior ofre-calculating a route to find an alternative route. Therefore, both theroute update and the route convergence may be referred to as routeswitching. The route update can enable all routers in a routing domainto achieve consistency in terms of a current network structure and routeforwarding.

FIG. 1 is a schematic structural diagram of a network system accordingto an embodiment of this application. The network system includes: anRTA, an RTB, an R1, an R2, an R3, and an R4.

The RTA and the RTB are ASBRs. The R1, the R2, the R3, and the R4 arerouters in an OSPF network area.

The RTA and the RTB respectively lead routes of an external network toOSPF routing tables of the RTA and the RTB, and respectively generateAS-external-LSA packets and send the AS-external-LSA packets to the R1,the R2, the R3, and the R4. In addition, the RTA, the RTB, the R1, theR2, the R3, and the R4 each generate one router link state advertisement(English: Router-LSA) packet and send the router link stateadvertisement packet to all neighbors.

In an OSPF network, different types of LSAs are defined in an OSPFprotocol based on different functions of the LSAs.

The Router-LSA is a type 1 LSA. The Router-LSA packet is used todescribe a link state and a distance value of the routers in the OSPFnetwork area, and flooding is performed only in one OSPF network area. Aformat of the Router-LSA packet is shown in FIG. 2.

The Router-LSA packet includes an LSA packet header and content of theRouter-LSA packet.

Fields shown in the LSA packet header have the following meanings:

LS age (link state age) field: Used to identify a time that passes byafter the LSA is generated.

Options (options) field.

LS type (link state type) field, Link State ID (link state identifier)field, and LS sequence number (link state sequence number) field: Usedtogether to determine whether these three fields describe a same LSA.

Link State ID field in the Router-LSA: Set as an OSPF router identifier.

Advertising router (advertising router) field: Used to identify a routerthat originates the LSA packet.

LS checksum (link state checksum) field: Used to identify a checksum offields other than the LS age field.

Length (length) field: Used to identify a total length of the LSApacket, including the LSA packet header.

Fields shown in the content of the Router-LSA packet have the followingmeanings:

#links (quantity of links) field: Used to identify a quantity of piecesof link information described in the LSA packet, including informationof all links and interfaces of a router in a specific area.

Link ID (link identifier) field: Used to identify a target connected toa router.

Link Data (link data) field: Used to identify link data. A value of thisfield depends on a link type.

Type (type) field: Used to describe a type of an interface networkconnected to a router. Four link types are described in the Router-LSApacket: A first type is a point-to-point link to another router; asecond type is a link to a transit network; a third type is a link to astub network; and a fourth type is a virtual link.

#TOS (quantity of types of services) field: Used to identify a quantityof different TOSs given for a link.

Metric (a value of overheads) field: Used to identify a value ofoverheads of a link.

The AS-external-LSA is a type 5 LSA. The AS-external-LSA packet is usedto describe a route from a router to an external network of an AS, andflooding is performed in the entire AS. A format of the AS-external-LSApacket is shown in FIG. 3. The AS-external-LSA packet includes an LSApacket header and content of the AS-external-LSA packet.

In the AS-external-LSA packet, a link state ID field in the LSA packetheader is used to identify a destination prefix address advertised bythe router to another router. Meanings of other fields in the LSA packetheader are the same as meanings of same fields shown in FIG. 2. Detailsare not described herein again.

Fields shown in the content of the AS-external-LSA packet have thefollowing meanings:

Network Mask (network mask) field: Used to identify a mask of theadvertised destination prefix address.

Forwarding address (forwarding address) field: Used to identify anaddress to which a packet sent to the advertised destination prefixaddress is forwarded.

Based on the schematic diagram of the application scenario shown in FIG.1, FIG. 4 is a schematic flowchart of a method for updating a route in anetwork according to an embodiment of this application. The networkincludes a first network device, a second network device, and a thirdnetwork device. The third network device is connected to the firstnetwork device and the second network device.

The connection between the third network device and the first networkdevice and the connection between the third network device and thesecond network device may be physical connections or may beprotocol-based connections. For example, if a distance between the thirdnetwork device and the first network device and a distance between thethird network device and the second network device are relatively short,the connections may be made by using physical links. If a distancebetween the third network device and the first network device and adistance between the third network device and the second network deviceare relatively long, for example, the third network device and the firstnetwork device are not in a same city or the third network device andthe second network device are not in a same city, the connections may beset up by using an OSPF protocol.

The method for updating a route in a network includes:

S401: The first network device sets up a correspondence between an IPnetwork segment and a first IP address.

The first IP address is an IP address of the first network device.

For example, the first network device may be the RTA or the RTB shown inFIG. 1, and the IP address of the first network device is 1.1.1.1. Anexternal network connected to the RTA or the RTB has three IP networksegments. A route prefix address of a first IP network segment is10.1.0.0. A route prefix address of a second IP network segment is10.2.0.0. A route prefix address of a third IP network segment is10.3.0.0.

For example, the first network device sets up a correspondence betweenthe IP address 1.1.1.1 and the route prefix address 10.1.0.0 of thefirst IP network segment.

S402: The first network device sends a first LSA packet to the thirdnetwork device.

The first LSA packet includes a second IP address. The first LSA packetis used to trigger the third network device to generate a first routewhose destination address is the second IP address. A next-hop IPaddress of the first route is the first IP address.

Optionally, there may be one or more first LSA packets.

The second IP address belongs to the IP network segment. For example, ifthe IP network segment is the first IP network segment, the second IPaddress may be an address belonging to the first IP network segment,such as 10.1.1.1, 10.1.2.0, or the like. If the IP network segment isthe second IP network segment, the second IP address may be an addressbelonging to the second IP network segment, such as 10.2.0.0, 10.2.1.1,or the like. If the IP network segment is the third IP network segment,the second IP address may be an address belonging to the third IPnetwork segment, such as 10.3.0.0, 10.3.2.1, or the like.

For example, if the first IP address is 1.1.1.1, the first networkdevice sets up a correspondence between the first IP address 1.1.1.1 andthe first IP network segment. If the second IP address belongs to thefirst IP network segment, for example, the second IP address is10.1.9.9, the first network device also sets up a correspondence betweenthe first IP address and the second IP address.

For example, when the first network device is the RTA shown in FIG. 1,the first LSA packet is a first AS-external-LSA packet. Based on theformat of the AS-external-LSA packet shown in FIG. 3, when generating acorresponding AS-external-LSA packet based on the correspondence that isset up between the IP network segment and the first IP address, thefirst network device sets a forwarding address field in the firstAS-external-LSA packet to 1.1.1.1.

S403: The third network device receives the first LSA packet sent by thefirst network device, and generates, based on the first LSA packet, afirst route whose destination address is a second IP address.

The third network device generates, based on the received first LSApacket, the first route whose destination address is the second IPaddress. A next-hop IP address of the first route is the first IPaddress.

Optionally, if the first LSA packet received by the third network deviceincludes a plurality of LSA packets, a corresponding quantity of routesare generated. All next-hop IP addresses of the generated routes are thefirst IP address.

For example, the third network device may be the R1, the R2, the R3, orthe R4 shown in FIG. 1. Based on the foregoing example, if the firstnetwork device is the RTA shown in FIG. 1, the third network device isthe R1, and the first LSA packet is the first AS-external-LSA packet,for the first route generated by the R1, a destination IP address of thefirst route is 10.1.9.9, and a next-hop IP address is 1.1.1.1. Duringpacket forwarding, first, the R1 sends a packet whose destination IPaddress is 10.1.9.9 to the RTA whose IP address is 1.1.1.1, and then theRTA forwards the packet to a network device whose IP address is10.1.9.9.

S404: The first network device sends a second LSA packet to the thirdnetwork device when determining that switching needs to be performed ona next hop of a route that is stored in the third network device, whosedestination IP address belongs to the IP network segment, and whosenext-hop IP address is the first IP address.

The first network device sends the second LSA packet to the thirdnetwork device when detecting that a packet forwarding function of thefirst network device is faulty, or when detecting that a CPU usage ofthe first network device exceeds a specific threshold, or when detectingthat a bandwidth usage of the first network device exceeds a specificthreshold.

The second LSA packet includes the first IP address. The second LSApacket is used to trigger the third network device to update, from thefirst IP address to a third IP address, the next-hop IP address of thefirst route. The third IP address is an IP address of the second networkdevice.

For example, if the first network device is the RTA shown in FIG. 1, thesecond network device is the RTB shown in FIG. 1. Route prefixes ofthree IP network segments connected to the first network device and thesecond network device are 10.1.0.0, 10.2.0.0, and 10.3.0.0. The firstnetwork device sets up a correspondence between the first IP address andthe route prefix 10.1.0.0, and the second LSA packet generated by thefirst network device is a first Router-LSA packet.

When the RTA generates the first Router-LSA packet, a quantity in a#links field is 1. When a correspondence is set up between an IP addressof the RTA and an IP network segment whose route prefix is 10.1.0.0, theIP address of the RTA corresponds to a section of packet content thatincludes a link ID field, a link data field, a type field, a #TOS field,and a metric field.

S405: The third network device receives the second LSA packet sent bythe first network device, and updates, from the first IP address to athird IP address, the next-hop IP address of the first route based onthe second LSA packet.

The third IP address is an IP address of the second network device.Based on the foregoing example, if the first network device is the RTAshown in FIG. 1, the second network device is the RTB shown in FIG. 1,the third network device is the R1, and the first IP address is 1.1.1.1.If the third IP address is 4.4.4.4, the third network device updates,from the first IP address to the third IP address, the next-hop IPaddress of the first route based on the second LSA packet, and in aspecific application example, updates, from 1.1.1.1 to 4.4.4.4, thenext-hop IP address of the first route. During packet forwarding, first,the R1 sends a packet whose destination IP address is 10.1.9.9 to theRTB whose destination address is 4.4.4.4, and then the RTB forwards thepacket to a network device whose IP address is 10.1.9.9.

In this embodiment of this application, when determining to update thenext-hop IP address of the route in the third network device, the firstnetwork device sends, to the third network device, the second LSA packetthat is used to trigger switching of the route's next-hop IP address tothe IP address of the second network device, so that the third networkdevice rapidly switches, to the IP address of the second network device,the next-hop IP address of the route that transmits the packet to thedestination IP address. This achieves a purpose of improving a speed ofroute switching.

Based on the method for updating a route in a network in the foregoingembodiment of this application, optionally, after the route switching,the method further includes: sending, by the first network device to thethird network device when the first LSA packet is stored in a link-statedatabase for a time length reaching a maximum age, a packet for deletingthe first LSA packet, so that the third network device deletes thestored first LSA packet. After sending the second LSA packet, the firstnetwork device may continue to perform an aging operation on the firstLSA packet in the foregoing manner, to delete the first LSA packet,thereby saving storage space.

Optionally, the foregoing method for updating a route further includesthe following steps:

The first network device sends a third LSA packet to the third networkdevice, where the third LSA packet includes the first IP address, andthe third LSA packet is used to trigger the third network device togenerate a route whose destination address is the first IP address.

The second network device sends a fourth LSA packet to the third networkdevice, where the fourth LSA packet includes the third IP address, andthe fourth LSA packet is used to trigger the third network device togenerate a route whose destination address is the third IP address.

The third network device receives the third LSA packet sent by the firstnetwork device, and generates, based on the third LSA packet, a firstroute whose destination address is a first IP address.

The third network device receives the fourth LSA packet sent by thesecond network device, and generates, based on the fourth LSA packet, aroute whose destination address is the third IP address.

The first network device sends a second LSA packet to the third networkdevice when determining that switching needs to be performed on a nexthop of a route that is stored in the third network device, whosedestination IP address belongs to the IP network segment, and whosenext-hop IP address is the first IP address. That the second LSA packetis used to trigger the third network device to update, from the first IPaddress to the third IP address, the next-hop IP address of the firstroute is specifically as follows:

The second LSA packet is used to instruct the third network device todelete the route whose destination address is the first IP address, totrigger the third network device to update the first route whosenext-hop IP address is the first IP address, so that the next-hop IPaddress of the first route is updated from the first IP address to thethird IP address, where the third network device stores a route whosedestination address is the third IP address.

The third network device updates, based on the second LSA packet, thefirst route whose next-hop IP address is the first IP address, so thatthe next-hop IP address of the first route is updated from the first IPaddress to the third IP address.

Optionally, the first IP address is an IP address of a logical interfaceof the first network device, and a forwarding address in the first LSApacket is the first IP address. A correspondence may be set up betweenthe logical interface and one network segment, or a correspondence maybe set up between the logical interface and a plurality of networksegments. The first IP address includes a plurality of IP addresses. TheIP address may be an IP address in a reserved network segment.

For example, the first network device may be the RTA or the RTB shown inFIG. 1.

For example, the first IP network segment and the second IP networksegment are grouped into a route group (English: group) 1, and the thirdIP network segment is grouped to a group 2. The group 1 includes routeprefix addresses 10.1.0.0 and 10.2.0.0. The group 2 includes a routeprefix address 10.3.0.0.

It should be noted that this embodiment of this application does notlimit a quantity of obtained route groups or limit a quantity of routeprefix addresses in an IP network segment that are included in the routegroups. Flexible grouping may be performed based on a specificrequirement.

Based on the foregoing obtained route groups, each route group iscorresponding to one logical interface. Each logical interface iscorresponding to one IP address. Optionally, the logical interface maybe a router loopback (English: Loopback) interface.

The first network device sets up a correspondence between the group 1and a loopback interface 1. An IP address of the loopback interface 1 is1.1.1.1. The first network device sets up a correspondence between thegroup 2 and a loopback interface 2. An IP address of the loopbackinterface 2 is 2.2.2.2.

For example, the first network device is the RTA shown in FIG. 1, andthe first LSA packet is an AS-external-LSA packet. Based on a format ofthe AS-external-LSA packet shown in FIG. 3 and a correspondence betweena group and a loopback, when generating the AS-external-LSA packet, theRTA determines an IP address of a logical interface corresponding to theroute group. A forwarding address field in the AS-external-LSA packet isset to the IP address of the logical interface, and a link state IDfield in the AS-external-LSA packet is set to a route prefix address ina currently based IP network segment.

The RTA generates a corresponding first AS-external-LSA packet based ona route prefix address 10.1.0.0 in the first IP network segment. Aforwarding address field in the first AS-external-LSA packet is set to1.1.1.1, and a link state ID field is set to 10.1.0.0.

The RTA generates a corresponding second AS-external-LSA packet based ona route prefix address 10.2.0.0 in the second IP network segment. Aforwarding address field in the second AS-external-LSA packet is set to1.1.1.1, and a link state ID field is set to 10.2.0.0.

The RTA generates a corresponding third AS-external-LSA packet based ona route prefix address 10.3.0.0 in the third IP network segment. Aforwarding address field in the third AS-external-LSA packet is set to2.2.2.2, and a link state ID field is set to 10.3.0.0.

For example, the first network device is the RTA shown in FIG. 1, andthe second LSA packet is a Router-LSA packet. Based on a format of theRouter-LSA packet shown in FIG. 2, with reference to the route groups towhich the route prefix addresses of all the foregoing IP networksegments belong, when the first network device generates a correspondingfirst Router-LSA packet based on an obtained route group, as describedby the first network device, a quantity in a #links field is a quantityof links that need to be set up between the current first network deviceand an external network segment. A link ID field, a link data field, atype field, a #TOS field, and a metric field are link informationcorresponding to a logical interface of the route group.

There is a restriction for the network device to process a forwardingaddress in a route calculation process. The restriction is as follows:If a forwarding address of the AS-external-LSA packet is non-zero, arouting table is searched for a route that is corresponding to theforwarding address. A matched routing entry needs to be an intra-arearoute or an inter-area route in an OSPF area. If no match is found, adestination prefix address of the LSA advertisement is not calculated.If a matched route can be found, the forwarding address is set to a nexthop of a destination prefix address.

Therefore, when the first network device generates the first Router-LSApacket, a type field included in the link information of the logicalinterface is set to the third type, that is, the link type is a link toa stub network.

For example, when the first network device is the RTA shown in FIG. 1, aconnected external network has three IP network segments, and routeprefix addresses of the three IP network segments are 10.1.0.0,10.2.0.0, and 10.3.0.0. For example, 10.1.0.0 and 10.2.0.0 belong to agroup 1, and an IP address of a loopback interface 1 corresponding tothe group 1 is 1.1.1.1; 10.3.0.0 belongs to a group 2, and an IP addressof a loopback interface 2 corresponding to the group 2 is 2.2.2.2.

When the RTA generates the first Router-LSA packet, a quantity in a#links field is 2. Each loopback interface is corresponding to a sectionof packet content that includes a link ID field, a link data field, atype field, a #TOS field, and a metric field.

When the first network device is the RTB shown in FIG. 1, a process ofgenerating the corresponding first Router-LSA packet based on the routegroup is the same as a process in which the RTA generates the firstRouter-LSA packet. Details are not described herein again.

For example, the third network device may be the R1, the R2, the R3, orthe R4 shown in FIG. 1.

The first route that reaches a destination IP address and that isgenerated by the third network device based on an AS-external-LSA packetincludes: a route whose next-hop address is 1.1.1.1 and a route whosenext-hop address is 2.2.2.2. The destination IP address is the second IPaddress and belongs to the IP network segment. For example, adestination address 10.1.9.9 belongs to the first IP network segment andbelongs to the group 1; a destination IP address 10.2.4.4 belongs to thesecond IP network segment and belongs to the group 1; and thedestination address 10.3.2.2 belongs to the third IP network segment andbelongs to the group 2.

When the first network device determines that a next-hop address of aroute from the third network device to the destination IP address needsto be updated from the IP address of the first network device to the IPaddress of the second network device, optionally, the first networkdevice may exchange link state data through flooding (English: Flooding)method.

For example, in an OSPF network area, flooding indicates that a routertransfers an LSA packet of the router to all of its neighboring OSPFrouters, and the neighboring routers update their respective databasesbased on received link state information, and forwards the link stateinformation to their neighboring routers. When a network isre-stabilized, in other words, after an OSPF routing protocol isupdated, all the routers calculate their respective routing tables basedon their respective link state information databases. The routing tableincludes costs spent when the packet is transferred from the router toeach reachable destination and a next router to which the packet isforwarded before reaching the destination.

For example, the first network device is the RTA shown in FIG. 1, thesecond network device is the RTB shown in FIG. 1, the third networkdevice is the R1 shown in FIG. 1, and the R1 has 10000 routes to anetwork segment corresponding to a route prefix address 10.3.0.0. TheRTA needs to update, to the RTB, the 10000 routes on the R1 that reachthe network segment corresponding to the route prefix address 10.3.0.0.

In this case, the RTA floods first Router-LSA packets that arecontinuously sent to the R1. Based on the foregoing examples, the RTAdetermines that the destination IP address 10.3.2.2 belongs to the group2, and an interface corresponding to the group 2 is a loopback interface2. Therefore, the flooding performed by the RTA is updating, to an IPaddress of the RTB, an IP address of the loopback interface 2 in thefirst Router-LSA packet, to generate a second Router-LSA packet.

The R1 re-calculates a route based on the second Router-LSA packetobtained after the first Router-LSA packet is updated. Because thesecond Router-LSA packet does not include the IP address of the loopbackinterface 2, but includes the updated IP address of the RTB, there-calculated route no longer includes a route to an area of 2.2.2.2,and no route to the destination IP address 10.3.2.2 is calculated again,but the destination IP address 10.3.2.2 is updated to the RTB.

In this embodiment of this application, the first network devicecommunicating with the external network divides the route prefixaddresses in the IP network segments into route groups, to flexiblyadjust a route in a network system, and improve flexibility of networkdeployment. In addition, the second LSA packet sent to the third networkdevice in the network area is flooded, so that the third network deviceswitches, to the IP address of the second network device, the next-hopIP address of the route whose next-hop IP address is the first IPaddress. In this way, one LSA packet is sent to enable the third networkdevice to switch, from the first network device to the second networkdevice, the next hop of the route belonging to the IP network segment,and there is no need to send an LSA packet for each to-be-updated route.This avoids occupation of excessive bandwidths during a route update andimproves a speed of the route update.

Based on the method for updating a route in a network in the foregoingembodiment of this application, an embodiment of this applicationfurther correspondingly discloses a network device for performing themethod for updating a route in a network.

As shown in FIG. 5, a first network device 50 disclosed in an embodimentof this application, a second network device, and a third network deviceare in a same network. The third network device is connected to thefirst network device and the second network device. The first networkdevice 50 includes:

a processing unit 502, configured to set up a correspondence between anIP network segment and a first IP address, where the first IP address isan IP address of the first network device; and

a sending unit 501, configured to send a first LSA packet to the thirdnetwork device, where the first LSA packet includes the second IPaddress, the first LSA packet is used to trigger the third networkdevice to generate a first route whose destination address is the secondIP address, the second IP address belongs to the IP network segment, anda next-hop IP address of the first route is the first IP address.

The processing unit 502 is configured to determine whether switchingneeds to be performed on a next hop of a route that is stored in thethird network device, whose destination IP address belongs to the IPnetwork segment, and whose next-hop IP address is the first IP address.

The sending unit 501 is configured to send a second LSA packet to thethird network device when the processing unit determines that switchingneeds to be performed on the next hop of the route that is stored in thethird network device, whose destination IP address belongs to the IPnetwork segment, and whose next-hop IP address is the first IP address,where the second LSA packet includes the first IP address, the secondLSA packet is used to trigger the third network device to update, fromthe first IP address to a third IP address, the next-hop IP address ofthe first route, and the third IP address is an IP address of the secondnetwork device.

Optionally, the sending unit 501 is further configured to send a thirdLSA packet to the third network device, where the third LSA packetincludes the first IP address, and the third LSA packet is used totrigger the third network device to generate a route whose destinationaddress is the first IP address.

Correspondingly, the second LSA packet is used to instruct the thirdnetwork device to delete the route whose destination address is thefirst IP address, to trigger the third network device to update thefirst route whose next-hop IP address is the first IP address, so thatthe next-hop IP address of the first route is updated from the first IPaddress to the third IP address, where the third network device stores aroute whose destination address is the third IP address.

For execution processes of the units in the first network devicedisclosed in the foregoing embodiment of this application, refer to thedescription related to the first network device in the foregoing methodfor updating a route in a network in the embodiments of thisapplication. A principle and an execution manner in this embodiment arethe same as those in the foregoing embodiments. Details are notdescribed herein again.

Based on the method for updating a route in a network in the foregoingembodiment of this application, an embodiment of this applicationfurther correspondingly discloses a third network device for performingthe method for updating a route in a network.

FIG. 6 shows a third network device 60 according to an embodiment ofthis application, including:

a receiving unit 601, configured to: receive a first LSA packet sent bythe first network device, where the first LSA packet includes a secondIP address, the second IP address belongs to the Internet Protocol IPnetwork segment; and receive a second LSA packet sent by the firstnetwork device, where the second LSA packet is sent to the third networkdevice when the first network device determines that switching needs tobe performed on a next hop of a route that is stored in the thirdnetwork device, whose destination IP address belongs to the IP networksegment, and whose next-hop IP address is the first IP address, and thesecond LSA packet includes the first IP address; and

a processing unit 602, configured to generate, based on the first LSApacket, a first route whose destination address is the second IPaddress, and a next-hop IP address of the first route is the first IPaddress, and update, from the first IP address to a third IP address,the next-hop IP address of the first route based on the second LSApacket, where the third IP address is an IP address of the secondnetwork device.

Optionally, the receiving unit 601 is further configured to receive athird LSA packet sent by the first network device, where the third LSApacket includes the first IP address. The processing unit 602 is furtherconfigured to generate, based on the third LSA packet, a route whosedestination address is the first IP address.

Correspondingly, the receiving unit 601 is further configured to receivea fourth LSA packet sent by the second network device, where the fourthLSA packet includes the third IP address.

The processing unit 602 is further configured to generate, based on thefourth LSA packet, a route whose destination address is the third IPaddress.

For execution processes of the units in the third network devicedisclosed in the foregoing embodiment of this application, refer to thedescription related to the third network device in the foregoing methodfor updating a route in a network in the embodiments of thisapplication. A principle and an execution manner in this embodiment arethe same as those in the foregoing embodiments. Details are notdescribed herein again.

With reference to the method for updating a route in a network in theembodiments of this application, the first network device and the thirdnetwork device disclosed in the embodiments of this application may alsobe implemented directly by using hardware, a memory executed by aprocessor, or a combination thereof.

Therefore, based on the method for updating a route in a network in FIG.4, an embodiment of this application further correspondingly discloses afirst network device 70 shown in FIG. 7 and a third network device 80shown in FIG. 8. The first network device 70 shown in FIG. 7 may be thefirst network device in the embodiment corresponding to FIG. 4 and canperform the method that is for updating a route in a network and that isrelated to the first network device in FIG. 4 and the foregoingembodiments. The third network device 80 shown in FIG. 8 may be thethird network device in the embodiment corresponding to FIG. 4 and canperform the method that is for updating a route in a network and that isrelated to the third network device in FIG. 4 and the foregoingembodiments.

As shown in FIG. 7, the network device 70 includes: a memory 701, a bus702, a processor 703, and an interface 704. The interface 704 may beimplemented in a wireless or wired manner, and specifically, may be acomponent such as a network interface card. The memory 701, theprocessor 703, and the interface 704 are connected by using the bus 702.

The interface 704 may specifically include a transmitter and a receiver,and may be configured to receive and transmit a packet between the firstnetwork device and the foregoing third network device in the foregoingembodiments of this application. For a detailed process, refer to acorresponding part related to packet receiving and sending of the firstnetwork device in the foregoing embodiments of this application. Detailsare not described herein again.

The processor 703 is configured to perform a route update performed bythe first network device in a network in the foregoing embodiments. Fora detailed process, refer to the foregoing embodiments of thisapplication. Details are not described herein again.

The memory 701 includes an operating system and an application program,and is configured to store an operation program, code, or an instructionfor updating a route in a network. When the processor 703 or a hardwaredevice updates a route, these programs, code, or instructions can beinvoked and executed to complete the related process in which the firstnetwork device updates the route in FIG. 4 and the foregoingembodiments. For a detailed process, refer to a corresponding part inthe foregoing embodiments of this application. Details are not describedherein again.

It may be understood that FIG. 7 merely shows a simplified design of thefirst network device. In an actual application, the first network devicemay include any quantity of interfaces, processors, memories, and thelike. All first network devices that can implement this application fallwithin the protection scope of this application.

In addition, an embodiment of this application provides a computerstorage medium, which is configured to store a computer softwareinstruction used by the foregoing first network device and includes arelated program used to perform FIG. 4 and the foregoing embodiments.

FIG. 8 is a schematic diagram of a hardware structure of a third networkdevice 80 according to an embodiment of this application. The thirdnetwork device 80 shown in FIG. 8 may perform corresponding stepsperformed by the third network device in the foregoing embodiments ofthis application.

As shown in FIG. 8, the third network device 80 includes: a memory 801,a bus 802, a processor 803, and an interface 804. The interface 804 maybe implemented in a wireless or wired manner, and specifically, may be acomponent such as a network interface card. The memory 801, theprocessor 803, and the interface 804 are connected by using the bus 802.

The interface 804 may specifically include a transmitter and a receiver,and may be configured to receive and transmit a packet between the thirdnetwork device and each of the foregoing first network device and secondnetwork device in the foregoing embodiments of this application. Forexample, for a detailed process of the interface 804, refer to acorresponding part about packet receiving and sending between the thirdnetwork device and each of the first network device and the secondnetwork device in the foregoing embodiments of this application. Detailsare not described herein again.

The processor 803 is configured to perform the route update performed bythe third network device in the foregoing embodiments. For example, theprocessor 803 is configured to perform a corresponding operation basedon a received LSA packet. For a detailed process, refer to acorresponding part in the foregoing embodiments of this application.Details are not described herein again.

The memory 801 includes an operating system and an application program,and is configured to store a program, code, or an instruction forupdating a route. When the processor 803 or a hardware device updates aroute, these programs, code, or instructions can be invoked and executedto complete the processing process related to the third network devicein FIG. 4 and the foregoing embodiments. For a detailed process, referto a corresponding part in the foregoing embodiments of thisapplication. Details are not described herein again.

It may be understood that FIG. 8 merely shows a simplified design of thethird network device. In an actual application, the third network devicemay include any quantity of interfaces, processors, memories, and thelike. All third network devices that can implement this application fallwithin the protection scope of this application.

In addition, an embodiment of this application provides a non-volatilecomputer readable storage medium, which is configured to store acomputer software instruction used by the foregoing third network deviceand includes a related program used to perform FIG. 4 and the foregoingembodiments.

FIG. 9 shows a network system 90 used for implementing a method forupdating a route in a network according to an embodiment of thisapplication. The network system 90 mainly includes a first networkdevice 901, a second network device 902, and a third network device 903.

In the network system disclosed in this embodiment of this application,the first network device 901 may be specifically the first networkdevice disclosed in FIG. 5 and FIG. 7. For a detailed process andexecution principle, refer to the foregoing description. Details are notdescribed herein again.

The second network device 902 is configured to forward a packet sent bythe third network device 903. Optionally, the second network device 902and the first network device 901 have a same function.

In the network system disclosed in this embodiment of this application,the third network device 903 may be specifically the third networkdevice disclosed in FIG. 6 and FIG. 8. For a detailed process andexecution principle, refer to the foregoing description. Details are notdescribed herein again.

In conclusion, according to the method for updating a route in a networkin the embodiments of this application, the first network device sends afirst LSA packet to the third network device, so that the third networkdevice generates a first route whose destination address is a second IPaddress, where a next-hop IP address of the first route is an IP addressof the first network device, and the second IP address belongs to an IPnetwork segment. When determining to update a next-hop IP address of aroute in the third network device, the first network device sends, tothe third network device, a second LSA packet that is used to triggerswitching of the route's next-hop IP address to an IP address of thesecond network device, so that the third network device rapidlyswitches, to the IP address of the second network device, the next-hopIP address of the route whose next-hop IP address is a first IP address.In this way, one LSA packet is sent to enable the third network deviceto switch, from the first network device to the second network device,the next hop of the route belonging to the IP network segment, and thereis no need to send an LSA packet for each to-be-updated route. Thisavoids occupation of excessive bandwidths during a route update andimproves a speed of the route update.

The embodiments in this specification are all described in a progressivemanner, for same or similar parts in the embodiments, refer to theseembodiments, and each embodiment focuses on a difference from otherembodiments. Especially, apparatus and system embodiments are basicallysimilar to a method embodiment, and therefore are described briefly; forrelated parts, refer to partial descriptions in the method embodiment.

Finally, it should be noted that the foregoing embodiments are merelyintended to describe the technical solutions of this application as anexample, but not to limit this application. Although this applicationand benefits of this application are described in detail with referenceto the foregoing embodiments, persons of ordinary skill in the artshould understand that they may still make modifications to thetechnical solutions described in the foregoing embodiments or makeequivalent replacements to some technical features thereof, withoutdeparting from the scope of the claims of this application.

What is claimed is:
 1. A method for updating a route in a network, themethod comprising: sending, by a first network device of the network, afirst link state advertisement (LSA) packet to a third network device ofthe network, wherein the first LSA packet comprises a second internetprotocol (IP) address that belongs to an IP network segment, and is totrigger the third network device to generate a first route whosedestination address is the second IP address, and whose next-hop IPaddress is a first IP address that is an IP address of the first networkdevice; and sending, by the first network device, a second LSA packet tothe third network device, wherein the second LSA packet comprises thefirst IP address, and is to trigger the third network device to updatethe next-hop IP address of the first route to a third IP address that isan IP address of a second network device of the network, wherein thethird network device is connected to the first network device and thesecond network device of the network.
 2. The method according to claim1, the method further comprising: setting up, by the first networkdevice, a correspondence between the IP network segment and the first IPaddress, wherein the IP network segment that the second IP addressbelongs to is different from an IP network segment that an IP address ofthe third network device belongs to.
 3. The method according to claim 2,the method further comprising: the second LSA packet is sent when thefirst network device determines that switching needs to be performed ona next hop of a route that is stored in the third network device, whosedestination IP address belongs to the IP network segment that the secondIP address belongs to, and whose next-hop IP address is the first IPaddress.
 4. The method according to claim 3, the method furthercomprising: sending, by the first network device, a third LSA packet tothe third network device, wherein the third LSA packet comprises thefirst IP address, and is to trigger the third network device to generatea route whose destination address is the first IP address; andcorrespondingly, wherein the second LSA packet is used to trigger thethird network device to update the next-hop IP address of the firstroute to the third IP address comprises: instructing, by the second LSApacket, the third network device to delete the route whose destinationaddress is the first IP address, to trigger the third network device toupdate the first route whose next-hop IP address is the first IPaddress, so that the next-hop IP address of the first route is updatedfrom the first IP address to the third IP address, wherein the thirdnetwork device stores a route whose destination address is the third IPaddress.
 5. The method according to claim 2, wherein: the first IPaddress is an IP address of a logical interface of the first networkdevice, and a forwarding address in the first LSA packet is the first IPaddress.
 6. The method according to claim 3, wherein: the first IPaddress is an IP address of a logical interface of the first networkdevice, and a forwarding address in the first LSA packet is the first IPaddress.
 7. A route switching method in a network, the methodcomprising: receiving, by a third network device of the network, a firstlink state advertisement (LSA) packet sent by a first network device ofthe network, wherein the first LSA packet comprises a second internetprotocol (IP) address that belongs to a IP network segment; generating,by the third network device based on the first LSA packet, a first routewhose destination address is the second IP address, and whose next-hopIP address is a first IP address that is an IP address of the firstnetwork device; receiving, by the third network device, a second LSApacket sent by the first network device, wherein the second LSA packetcomprises the first IP address; and updating, by the third networkdevice, the next-hop IP address of the first route to a third IP addressbased on the second LSA packet, wherein the third IP address is an IPaddress of a second network device of the network, wherein the thirdnetwork device is connected to the first network device and the secondnetwork device.
 8. The method according to claim 7, wherein the secondLSA packet is received from the first network device when the firstnetwork device determines that switching needs to be performed on a nexthop of a route that is stored in the third network device, whosedestination IP address belongs to the IP network segment, and whosenext-hop IP address is the first IP address.
 9. The method according toclaim 8, the method further comprising: receiving, by the third networkdevice, a third LSA packet sent by the first network device, wherein thethird LSA packet comprises the first IP address; and generating, by thethird network device based on the third LSA packet, a route whosedestination address is the first IP address; and correspondingly,wherein the updating, by the third network device, the next-hop IPaddress of the first route to a third IP address based on the second LSApacket comprises: receiving, by the third network device, a fourth LSApacket sent by the second network device, wherein the fourth LSA packetcomprises the third IP address; generating, by the third network devicebased on the fourth LSA packet, a route whose destination address is thethird IP address; and updating, by the third network device based on thesecond LSA packet, the first route whose next-hop IP address is thefirst IP address, so that the next-hop IP address of the first route isupdated from the first IP address to the third IP address.
 10. Themethod according to claim 8, wherein: the first IP address is an IPaddress of a logical interface of the first network device, and aforwarding address in the first LSA packet is the first IP address. 11.The method according to claim 9, wherein: the first IP address is an IPaddress of a logical interface of the first network device, and aforwarding address in the first LSA packet is the first IP address. 12.A first network device in a network, comprising: a memory storinginstructions; and a processor coupled to the memory, wherein theinstructions are executed by the processor to cause the first networkdevice to: send a first link state advertisement (LSA) packet to a thirdnetwork device of the network, wherein the first LSA packet comprises asecond internet protocol (IP) address that belongs to an IP networksegment, wherein the first LSA packet is to trigger the third networkdevice to generate a first route whose destination address is the secondIP address, and whose next-hop IP address is a first IP address that isan IP address of the first network device; and send a second LSA packetto the third network device, wherein the second LSA packet comprises thefirst IP address, wherein the second LSA packet is to trigger the thirdnetwork device to update the next-hop IP address of the first route to athird IP address that is an IP address of a second network device of thenetwork, wherein the third network device is connected to the firstnetwork device and the second network device.
 13. The first networkdevice according to claim 12, wherein the instructions are executed bythe processor to further cause the first network device to: set up acorrespondence between the IP network segment and the first IP address,wherein the IP network segment is different from an IP network segmentthat an IP address of the third network device belongs to.
 14. The firstnetwork device according to claim 13, the instructions are executed bythe processor to further cause the first network device to: send a thirdLSA packet to the third network device, wherein the third LSA packetcomprises the first IP address, and is to trigger the third networkdevice to generate a route whose destination address is the first IPaddress; and correspondingly, wherein the second LSA packet is toinstruct the third network device to delete the route whose destinationaddress is the first IP address, to trigger the third network device toupdate the first route whose next-hop IP address is the first IPaddress, so that the next-hop IP address of the first route is updatedfrom the first IP address to the third IP address, wherein the thirdnetwork device stores a route whose destination address is the third IPaddress.
 15. The first network device according to claim 12, wherein thefirst IP address is an IP address of a logical interface of the firstnetwork device, and a forwarding address in the first LSA packet is thefirst IP address.
 16. The first network device according to claim 13,wherein the first IP address is an IP address of a logical interface ofthe first network device, and a forwarding address in the first LSApacket is the first IP address.
 17. A third network device in a network,the third network device comprising: a memory storing instructions; anda processor coupled to the memory, wherein the instructions are executedby the processor to cause the third network device to: receive a firstLSA packet sent by a first network device of the network, wherein thefirst LSA packet comprises a second IP address that belongs to a IPnetwork segment; generate, based on the first LSA packet, a first routewhose destination address is the second IP address, and whose next-hopIP address is a first IP address that is an IP address of the firstnetwork device; receive a second LSA packet sent by the first networkdevice, wherein the second LSA packet comprises the first IP address;and update the next-hop IP address of the first route to a third IPaddress based on the second LSA packet, wherein the third IP address isan IP address of a second network device of the network, wherein thethird network device is connected to the first network device and thesecond network device.
 18. The third network device according to claim17, wherein the second LSA packet is received from the first networkdevice when the first network device determines that switching needs tobe performed on a next hop of a route that is stored in the thirdnetwork device, whose destination IP address belongs to the IP networksegment, and whose next-hop IP address is the first IP address.
 19. Thethird network device according to claim 17, wherein the instructions areexecuted by the processor to further cause the first network device to:receive a third LSA packet sent by the first network device, wherein thethird LSA packet comprises the first IP address; generate, based on thethird LSA packet, a route whose destination address is the first IPaddress; receive a fourth LSA packet sent by the second network device,wherein the fourth LSA packet comprises the third IP address; generate,based on the fourth LSA packet, a route whose destination address is thethird IP address; and update, based on the second LSA packet, the firstroute whose next-hop IP address is the first IP address, so that thenext-hop IP address of the first route is updated from the first IPaddress to the third IP address.
 20. The third network device accordingto claim 17, wherein the first IP address is an IP address of a logicalinterface of the first network device, and a forwarding address in thefirst LSA packet is the first IP address.